Biaxial orientation of polyolefins, particularly polypropylenes, produces films which have applications in the polyolefin film business, for example snack food packaging, cigarette overwrap, electronic components wrapping, packaging tape, and shrink film. The polymers normally employed in the preparation of biaxially oriented films are isotactic homopolymers with high stereoregularity, although on some occasions the use of syndiotactic polymers has been proposed. Also suitable are co-polymers of isotactic polypropylenes with a small content of ethylene (mini-random co-polymers).
Polyolefin films can be prepared as opaque films rather than just as transparent films. In some applications, opacity is a desirable property for such packaging films. Opacity can protect materials from being degraded by light. For example, packaged foodstuff can be subject to deterioration caused by exposure to light, particularly light having a wavelength of up to about 450 nm. Even when a degree of opacity is present in the film, spoilage may occur if the film allows passage of too much light, therefore highly opaque films are the most desirable for these purposes.
The major end-use applications in food packaging were: snack food packaging, bakery products, candy packaging, cheese packaging, coffee & tea packaging, cracker bags, pet food packaging, pasta packaging and others.
The other segment was industrial applications such as: pressure sensitive tapes, labels, capacitor films, audio/video cassettes & CD overwrap, cable insulation and others.
From these two principal markets, opaque films find their niche in two major applications: candy bar wrappers (food packaging application) and labels (industrial applications).
Achieving an opaque film made from polypropylene using resin modifiers is desirable. There exists a need to use materials other than the traditional cavitating agents to obtain opaque film. Such materials may include polymethylpentene and polytrimethylene terphthalate.